Most computer operating systems provide different privilege or priority levels for different functions performed by software. In the IBM OS/2 operating system, file management functions are most effectively performed at the highest priority level (ring zero) while application (user programs) operate at a lower priority level (ring 3). Under ordinary circumstances this poses no significant problem. However, kernel programs, operating at ring zero, can not communicate directly with externally located programs such as remote file server programs.
When porting an existing file server system such as the Andrews File Server (AFS) to run on the OS/2 system this limitation imposed operational restrictions since file operation requests from an application level program (ALP) to a file driver kernel level program (KLP) most often required communications with the remote file server.
An obvious solution was to provide a cache manager program operating at the application level in communication with the kernel level file driver program to support remote communications with the file server program. This solution, however, adversely impacts performance when existing communication methods are used for this purpose. An alternative method for this communication is needed if performance is not to be adversely impacted.